This invention pertains to photodetectors. In particular, the invention pertains to a high speed, efficient, low noise semiconductor photodetector employing a multilayer structure which allows for the separate characterization of device speed, efficiency and noise suppression in the various layers.
The highest-speed currently available semiconductor photodetectors are thin p-i-n photodiodes, where the (i) region is an intermediate undoped region which usually functions as the active photodetecting layer. The characteristics of this layer are such that it must be thin enough so that the transit of photogenerated carriers is fast. Also, the band gap of the layer must be sufficiently low so that photogenerated carriers are efficiently generated within the layer, but the band gap must be high enough so that thermally generated carriers do not produce an unacceptably high dark current. In currently available systems, one or more of these features (speed, efficiency, noise reduction) may be compromised in order to enhance some other feature.
For example, known Schottky barrier devices, such as Badoz et al., U.S. Pat. No. 5,140,381 and Archer, U.S. Pat. No. 3,757,123, have been employed in photodetectors. However, these devices include at least one metallic layer. The metal layer reduces the speed of such devices. Operation of these devices is limited to the infrared region.
Some devices, such as Hamana, U.S. Pat. No. 5,101,254, employ a valence band which has a notch for trapping minority carriers (e.g., holes). It is undesirable to trap holes in the transport region, because the holes attract majority carriers (e.g., electrons) and various device characteristics, including device speed are affected.
Some systems, e.g., Chen, U.S. Pat. No. 4,471,370, endeavor to increase the response time or speed of the device by depleting minority carriers from all the layers. Minority carriers (holes in this case) are less mobile than majority carriers (electrons in this case). Hence, the device is faster. Chen has a 600 picosecond (psec) fall time which limits its operation to about 1.7 GHz (10.sup.9 Hertz).
Some devices are operative only with highly restrictive materials constraints. For example, Pankove, U.S. Pat. No. 4,985,742, is limited to structures in which at least one layer is gallium nitride. Furthermore, Pankove requires that minority carriers be trapped thereby increasing electron current.
The present invention is related to real-space electron transfer devices. In such a device, carriers gain enough energy from an applied electric field to be able to pass over an energy barrier into another region of the device. Carriers in the present invention gain energy from incident light and are able to pass over a barrier region to a collector. There appear to be no other photodetectors wherein the excess energy of the carriers over the band gap of a barrier layer is supplied by the incident light.
Prior photoconductor devices are slow or inefficient or noisy. They are often quite complicated or limited in wavelength range, and the best available photodetectors are expensive. A currently available photodetector device employs dielectric mirrors on each side of a photodiode to reflect light internally. This device is difficult to fabricate and is, consequently, extremely expensive.
Currently available prior art devices have relatively low efficiencies in a range of about 25-50%. While efficiencies up to about 90% may be achieved at great expense, the frequency response of such devices is often limited. In general, the bandwidth of currently available devices extends to a maximum of about 100 GHz.